If you restored your iPhone 5c to the 6s the passcode will be the same. If you set it up as new and it's asking for 6 characters then that's what you will need to do as that's how it is set up to unlock. Did you set it up or did someone else?

Now if you're just trying to set it up with 4 instead of 6, there is an option on that screen to go to Options... You can choose 4 on that screen.

If you restored your iPhone 5c to the 6s the passcode will be the same. If you set it up as new and it's asking for 6 characters then that's what you will need to do as that's how it is set up to unlock. Did you set it up or did someone else?

Now if you're just trying to set it up with 4 instead of 6, there is an option on that screen to go to Options... You can choose 4 on that screen.